The present invention relates to a PLL (Phase-Locked Loop) frequency synthesizer used in, among other apparatuses, a radio telephone. In particular, the present invention relates to a PLL frequency synthesizer having a charge-pump circuit.
With the development of digital-signal processing technologies going on in recent years, there has arrived on the scene a PLL frequency synthesizer which adopts a digital-signal processing technology using rectangular waves in the processing to compare a reference signal with a PLL loop signal wave as a means for controlling the output of a VCO (Voltage Controlled Oscillator). Since a PLL of the PLL frequency synthesizer of this type in principle includes digital-signal processing based on rectangular waves, there are encountered a variety of problems that are not seen in a PLL based on the conventional analog-signal processing. In order to solve these problems, a number of studies of relevant technologies have been conducted.
One of the developed technologies is disclosed in U.S. Pat. No. 5,055,803. According to this technology, ranges of values of device constants of components composing a loop filter of a PLL frequency synthesizer adopting a digital-signal processing technology based on rectangular waves are prescribed in order to improve the stability of the PLL against changes in device sensitivity occurring in the components. To put it in detail, since the configuration of the PLL for carrying out digital-signal processing includes a charge-pump circuit, the degree of a transfer function prescribing the loop gain of the PLL increases by one over that of a transfer function of a PLL for carrying out analog-signal processing, giving rise to a new stability problem.
In the conventional technology described above, a means for controlling a spurious wave generated in a signal wave propagating around the PLL is disclosed. The spurious wave is newly developed as a result of inclusion of digital-signal processing in the PLL.
In the conventional technology described above, however, there is not given any consideration regarding spurious noise generated by an emission line spectrum superposed on a signal wave propagating around the PLL of the PLL frequency synthesizer carrying out digital-signal processing, giving rise to a problem that the amount of spurious noise is not sufficiently reduced.
In the PLL frequency synthesizer carrying out digital-signal processing, the PLL thereof includes an operation to convert time variations of a signal wave into variations in phase. This operation is equivalent to the Fourier transformation for transforming a signal wave from a time axis to a frequency axis. In the PLL frequency synthesizer carrying out digital-signal processing, since digital-signal processing is carried out, a rectangular-wave signal propagates around the PLL in principle. Thus, the Fourier transformation is performed on a rectangular wave. Since the rectangular wave propagating around the PLL has a fixed period, a Fourier transformation for a periodical rectangular wave is carried out in the PLL. Since the Fourier transformation for a rectangular wave exhibiting periodicity with time variations introduces a limited emission line spectrum subject to a frequency-band restriction, in the PLL frequency synthesizer carrying out digital-signal processing, a signal wave propagating around the PLL has the limited emission line spectrum superposed thereon on the frequency axis, giving rise to a problem that the limited emission line spectrum appears as spurious noise.